1. Field of the Invention
The present invention relates generally to a method of grinding a core blank for forming a magnetic head or heads, and more particularly to such a grinding method that permits the core blank to yield at least one head core each having a magnetic gap whose depth dimension is easily controlled to a predetermined value with high accuracy.
2. Discussion of the Prior Art
There are conventionally used various types of head cores each used for a magnetic head for a video tape recorder (VTR), floppy disk drive (FDD), rigid disk drive (RDD), digital audio tape (DAT), or other devices, each head core having an annular or closed magnetic path, and a magnetic gap formed in the magnetic path. Known examples of the head cores may include: a head core for a bulk-type magnetic head in which ferrite cores define a magnetic path; a head core for a thin-film type magnetic head in which magnetic thin films or layers define a magnetic path; a monolithic type head core having a slider as an integral part thereof; and a composite type head core having a slider as a separate body. Also known is a MIG (Metal-In-Gap) head core having a magnetic gap in which is provided a metallic magnetic film having a high magnetic flux density.
To meet with the recent tendency toward a high recording density of a magnetic recording medium such as a magnetic disk, the magnetic head cores as indicated above need to be fabricated with high processing accuracy. In particular, the depth dimension of the magnetic gap needs to be controlled to a predetermined or nominal value with high dimensional accuracy on the order of several microns, since the gap depth has a great influence on an electro-magnetic or magneto-electric conversion efficiency. The above-indicated depth dimension is construed as the depth of the magnetic gap which extends from a sliding surface of the head core that is opposed to a magnetic recording medium for sliding contact therewith, to an apex of a coil-winding groove formed in the head core, which apex is located remote from the sliding surface. This apex defines the innermost end of the magnetic gap.
After formation of a core blank which, after grinding and other finishing steps, provides at least one head core each having an annular magnetic path with a magnetic gap, the core blank is ground at its surface that is to be opposed to a magnetic recording medium, so as to establish the final depth dimension of the magnetic gap. During this grinding operation, it is extremely difficult to directly measure or observe the currently established gap depth at a side surface of the core blank perpendicular to the grinding surface thereof, since a core portion of a thin-film type magnetic head, for example, is usually covered by a thick protective layer having a thickness of several tens of microns.
In view of the above, a method of grinding a core blank for a magnetic head having a magnetic gap is disclosed in JP-A-53-142214. This method employs markers of a triangle or other shape, which are formed by photolithography in the core blank such that each marker has opposite edges that are spaced from each other in the direction of width of the magnetic gap, by a distance that continuously varies in the direction of depth of the gap. During the grinding operation, the distance between the opposite edges of each marker that is exposed on the grinding surface of the core blank is measured so as to determine the required depth of grinding that should be further effected to establish the nominal depth dimension of the magnetic gap, that is, the distance between the currently ground surface and a grinding depth line which defines the nominal gap depth. According to this method, therefore, the required residual grinding depth or amount can be easily determined.
However, a further study on the above grinding method by the present inventor revealed that the accuracy in measuring the distance between the opposite edges of each marker is unsatisfactory or poor, whereby it is difficult to establish the nominal depth dimension of the magnetic gap with sufficiently high accuracy. Namely, the distance between the opposite edges of the marker, i.e., the width dimension of the marker, must highly accurately correspond to the distance between the currently ground surface of the core blank and the above-indicated grinding depth line, which distance indicates the required residual depth of grinding that should be further effected on the core blank.
The marker used in the above method is formed by using a resist film that is formed by photolithography on a substrate or a thin film formed on the substrate. This resist film is patterned into the shape of the marker to be formed, so that the substrate or thin film is then processed by plating or etching to form the marker corresponding to the resist pattern. In this case, the dimensions of the resist pattern may deviate from the nominal values, due to variation of the thickness of the resist film, fluctuating conditions of development of the resist pattern, and other factors. Further, the dimensions of the marker eventually obtained may be shifted or differ from those of the resist pattern during the etching operation, as an etching depth or amount is varied depending upon etching conditions. Due to the pattern forming error or deviation as described above, it is extremely difficult to obtain a marker having intended dimensions with high reliability.
Consequently, there inevitably arises a change in the relationship between the width dimension of the marker and the distance between the currently ground surface of the core blank and the nominal grinding depth position, whereby the required residual depth of grinding to be further effected, which is determined based on the width dimension of the marker, cannot be relied upon to establish the nominal or intended depth dimension of the magnetic gap with sufficiently high accuracy.